1. Field of the Invention
The present invention relates to a jet-molding unit adapted for use in attaching soles directly to uppers, the jet-molding unit having a mixing head equipped with a worm gear designed to be movable to enable automatic exchange of the worm gear by appropriate means.
2. The Prior Art
The shoe manufacturing industry generally defines the process of directly attaching soles to uppers as the automatic production of soles and the simultaneous attachment of the soles to the uppers. This procedure is suitably accomplished by jet molding. Materials used for forming the uppers can include, for example, leather, textiles, or synthetic polymeric substances and for the soles suitable materials include polyvinylchloride, nylon, polyurethane granulate, liquid components of polyurethane and thermoplastic rubbers. The materials used for the production of soles are generally injected into closed molds.
Devices for directly attaching soles to finished uppers are known in the art. Especially well known are turntable systems or so-called rotators which have, for example, eighteen molding stations installed on a work table and which, while rotating, are caused to move sequentially into the axis of the jet-molding unit which then moves to a molding point for jet molding the sole to the upper. For multi-colored soles or soles composed of more than one part, two jet-molding units are provided which are positioned about the periphery of the turntable and at an angle to one another.
The jet-molding unit is moved into the molding position by means of a conveying cylinder located in front of the jet-molding unit form. When the molding material from the molding channel can no longer flow back, the jet-molding unit is moved into its rearward position and, in a turntable system, the rotation of the turntable for another cyclical turn is then initiated.
A jet-molding unit comprises primarily a nozzle-type mouthpiece, a mixing head, a worm gear and pumps. The mixing head, in turn, comprises a casing and a worm gear with its worm gear mounting and worm gear bearings. The mixing head with worm gear mounting are fixedly attached in the jet-molding unit.
When processing liquid polyurethane, two components generally are used which are conveyed by means of a dosing pump. A separate valve is provided for each component on the mixing head. The two components are thoroughly mixed in a so-called mixing chamber in the mixing head by means of the worm gear. The mixing chamber is generally designed to be conical in front and cylindrical on the rear and the mixing worm gear has a corresponding shape. The worm gear rotates at about 15,000 to 18,000 rpm. The mixing gap between the external contour of the worm gear and the internal contour of the mixing chamber is adjustable in size from about 0.5 to 1.5 mm. The worm gear is subsequently adjusted horizontally in the mixing head so that the mixing gap is now reduced to be only 0.1 mm. The axial movement of the worm gear is caused by a worm gear thrust cylinder. Because of the high rpm ratio of the worm gear and because of the dynamic pressure which is created in the mixing chamber as a result of its conical shape, the components are thoroughly mixed inside the mixing chamber.
Thereafter, the component mixture is caused to move into the jet-molding form through the conveying effect of the worm gear. When the mixing process is completed, the valves close. The worm gear is self-cleaning in the cleansing position because of the strong centrifugal force.
The known, state-of-the-art jet-molding units, however, have the disadvantage of only permitting a manual replacement of the worm gear, where the worm gear, after the mouthpiece is unscrewed, is removed from the front of the mixing head. When polyurethane (PUR) is used as injection material a replacement of the worm gear may be necessary every 1.5 to 2 hours since the threads of the worm gear become clogged which adversely affects the proper mixing of the components. Moreover, the manual replacement of the worm gear takes considerable time and also requires the presence of an operator which keeps this manual process from being cost effective. Furthermore, use of an auxiliary tool in removal could cause damage to the worm gear.